This disclosure relates to seismic exploration for oil and gas and, in particular but not by way of limitation, relates to the usage of seismic receivers (e.g. geophones or the like) as sources in order to generate information on near-surface structures of the Earth and/or for analyzing a layout/arrangement of devices (such as seismic receivers, seismic sources and including the seismic receivers being used as sources) in the seismic survey.
Seismic exploration may involve surveying subterranean geological formations for hydrocarbon deposits. A survey may involve deploying seismic source(s) and seismic receivers at predetermined locations. The seismic sources are used to generate seismic waves, which propagate into the geological formations, creating pressure changes and vibrations along the way. Changes in elastic properties of the geological formation scatter the seismic waves, changing properties of the waves, including the direction of propagation of the waves. In a seismic survey, part of the energy emitted by the seismic sources and scattered by the subterranean formations reaches the seismic receivers. Some seismic receivers are sensitive to pressure changes (e.g. hydrophones or the like) and others are sensitive to particle motion (e.g., geophones or the like); a seismic survey may include us one or both types of seismic receivers. In response to the scattered and received seismic waves, the seismic receivers generate electrical signals or the like, which signals comprise seismic data. The seismic data generated by the seismic receivers may be processed to produce an image of an interior section of the Earth, produce parameters representative of properties of an interior section of the Earth, which parameters may indicate the presence or absence of probable locations of hydrocarbon deposits, and/or the like.
Some seismic surveys are known as “marine” surveys because the survey is conducted in marine environments. “Marine” surveys may be conducted in saltwater environments, fresh water environments and brackish water environments. In one type of marine survey, called a “towed-array” survey, an array of one or more seismic sources and one or more streamers containing seismic sensors is towed behind a survey vessel.
Seismic surveys conducted on land are known as “land” surveys. FIG. 1 illustrates a land seismic survey layout, which will be described in more detail below. Land surveys may use dynamite, seismic vibrators and/or the like as seismic sources. In a land survey, arrays of seismic sensor-containing cables are laid on the ground to receive seismic signals generated by seismic sources and reflected, scattered and/or the like from subterranean formations/features. In the survey, seismic signals may be converted to electrical signals, digitized, stored and/or transmitted by the seismic sensors/receivers to data storage and/or processing facilities nearby, e.g. a recording truck. Land surveys may use wireless seismic receivers to avoid the limitations of cables. Some seismic surveys may be conducted in areas between the land and the sea, which is often referred to as the “transition zone”.
Seabed seismic survey, which may incorporate the use of both hydrophones and geophones, may be conducted on the seabed.
Depending on the survey environment, the sources used in the seismic survey may comprise airguns, waterguns, marine vibrators and/or the like for marine seismic surveys; vibrators, dynamite and/or the like for land seismic surveys. The receivers used in the seismic survey may also vary. For example, the seismic receivers may be geophones, which measure earth movement; the receivers may be hydrophones, which measure pressure waves (i.e. sound); or the receivers may be accelerometers, which measure the acceleration of particles in water or earth giving an indication/measurement of earth motion or pressure waves. In seismic surveys, the seismic receivers convert the changes in the measured physical parameters into electrical signals that can be digitized, processed and/or interpreted.
In a seismic survey, a geophone may be designed to generate a voltage in response to a movement of the Earth/an Earth formation.
FIG. 2 illustrates components of a geophone 200 for use in a seismic survey. The geophone 200 comprises a mass 210 that is suspended from a spring 215 and the spring 215 in turn is attached to a body 220 (e.g., the casing of the geophone or the like) that is coupled to a ground surface of the Earth 250. The mass 210 is surrounded by a magnet 230, and a conducting wire 240 is coiled around the mass 210, with the wire 240 forming an electrical circuit 245 including an electrical output sensor 260 (i.e., a voltmeter or the like). The electrical circuit 245 may also comprise an amplifier (not shown).
When the ground surface 250 moves, the mass 210 and the coiled conducting wire 240 move relative to the magnet 230. The movement of the coiled conducting wire 240 within the magnetic field of the magnet 230 produces an electrical current in the conducting wire 240, and this current can measured as an electrical output 270 by the electrical output sensor 260. The output voltage measured by the electrical output sensor 260 is proportional to the displacement of the ground surface 250.
A hydrophone is a device designed for use in detecting pressure changes (i.e. sound, acoustic waves and/or the like). Hydrophones may comprise transducers that convert sound signals/acoustic signals into electrical signals. Hydrophones may be used under water in a marine seismic survey during marine seismic acquisition. Hydrophones may be deployed in a seismic survey in a streamer, which comprises an elongated housing containing a plurality of hydrophones, that may be towed by a seismic vessel, deployed down a borehole and/or the like.
FIG. 3 illustrates a hydrophone 300. The hydrophone 300 comprises a pressure transducer 310 for converting pressure changes into electrical signals and a pair of wires 320 for outputting the electric signals indicating the changes in pressure.
An accelerometer is a device used in a seismic survey to measure an acceleration of an Earth surface, where the acceleration of the Earth surface is produced in response to the injection of seismic energy into the Earth by a seismic source, such as a vibrator or the like. An accelerometer converts the acceleration of an object into an electrical signal. An accelerometer may be used measure the acceleration of a particle in water, which measurement is indicative/provides a measurement of a sound/acoustic wave traveling in the water.
Two examples of accelerometers are shown in FIGS. 4a and 4b. FIG. 4a illustrates a Micro Electro-Mechanical System (“MEMS”) type accelerometer 401, in which most components are included inside an integrated circuit (“IC”) chip 450. The accelerometer 401 may be used to measure acceleration of an object in an X 410 direction, a Y 420 direction and a Z 430 direction and the measured accelerations may be read by a machine or a microprocessor (not shown) via an input/output interface 440.
FIG. 4b illustrates another type of accelerometer 402, which is similar to a geophone except that the output signal indicates the acceleration of an object rather than the displacement or the velocity of the object. In FIG. 4b, the accelerometer 402 measures acceleration in one direction 455, which direction is perpendicular to a base 460 of the accelerometer 402. The accelerometer 402 may comprise an internal Faraday shield 462, impedance matching electronics 465, inertial mass 470, sensing element 480 and a signal outlet 490.
One or more types of receivers may be used in a seismic survey to measure various characteristics of seismic waves generated in a seismic survey, the analysis of which may identify properties of subsurface earth structures, be used to generate images of subterranean formations and/or identify subsurface locations of valuable resources, such as hydrocarbons.